Reversible heat pumps are well known in the art and commonly used for cooling and heating a climate controlled comfort zone with a residence or a building. A conventional heat pump includes a compressor, a suction accumulator, a reversing valve, an outdoor heat exchanger with an associated fan, an indoor heat exchanger with an associated fan, an expansion valve operatively associated with the outdoor heat exchanger and a second expansion valve operatively associated with the indoor heat exchanger. The aforementioned components are typically arranged in a closed refrigerant circuit pump system employing the well known refrigerant vapor compression cycle. When operating in the cooling mode, excess heat absorbed by the refrigerant in passing through the indoor heat exchanger is rejected to the environment as the refrigerant passes through the outdoor heat exchanger.
It is well known in the art that an additional refrigerant-to-water heat exchanger may be added to a heat pump system to absorb this excess heat for the purpose of heating water, rather than simply rejecting the excess heat to the environment. Further, heat pumps often have non-utilized heating capacity when operating in the heating mode for heating the climate controlled zone. For example, each of U.S. Pat. Nos. 3,188,829; 4,098,092; 4,492,092 and 5,184,472 discloses a heat pump system including an auxiliary hot water heat exchanger. However, these systems do not include any device for controlling the refrigerant charge within the refrigerant circuit. Therefore, while functional, these systems would not be optimally efficient in all modes of operation.
In heat pump systems, the outdoor heat exchanger and the indoor heat exchanger each operate as evaporator, condenser or subcooler, depending on the mode and point of operation. As such, condensing may occur in either heat exchangers, and the suction line may be filled with refrigerant in a gaseous or liquid state. As a consequence, the amount of system refrigerant charge required in each mode of operation in order to ensure operation within an acceptable efficiency envelope will be different for each mode.
U.S. Pat. No. 4,528,822 discloses a heat pump system including an additional refrigerant-to-liquid heat exchanger for heating liquid utilizing the heat that would otherwise be rejected to the environment. The system is operable in four independent modes of operation: space heating, space cooling, liquid heating and simultaneous space cooling with liquid heating. In the liquid heating only mode, the indoor heat exchanger fan is turned off, while in the space cooling and liquid heating mode, the outdoor heat exchanger fan is turned off. A refrigerant charge reservoir is provided into which liquid refrigerant drains by gravity from the refrigerant to liquid heat exchanger during the liquid heating only mode and the simultaneous space cooling and liquid heating mode. However, no control procedure is disclosed for actively controlling refrigerant charge in the refrigerant circuit in all modes of operation. Further, no simultaneous space heating and liquid heating mode is disclosed.
Accordingly, it is desirable that heat pump system with liquid heating capability operate effectively in an air cooling only mode, an air cooling and liquid heating mode, an air heating only mode, an air heating and liquid heating mode, and a liquid heating only mode.